A memory device stores digital data in an array of memory cells. In order to read information from such an array, a cell is sensed for the presence or absence of charge. That information is amplified and transferred out of the memory device. Typically, an output driver coupled to or part of an output buffer is used to transfer the information out of the memory device.
One typical circuit arrangement for an output driver is a push-pull circuit. In such a circuit, a pull-up transistor is connected serially to a pull-down transistor between two potentials. The location in which the two transistors are connected, such as between drain of one and source of the other, typically provides a connection to an input/output pad (DQ pad).
To drive a logic high level output, a pull-up transistor is activated and a pull-down transistor is deactivated. As the pull-up transistor has its source connected to a logic high level, such as V.sub.cc, the DQ pad is driven toward the logic high level. When a logic low level is to be outputted, the pull-down transistor is activated and the pull-up transistor is deactivated. By activating the pull-down transistor, the DQ pad is coupled to a logic low level, such as ground or V.sub.ss, to provide a logic low level output.
A problem arises when either or both pull-up or pull-down transistor conduct at a subvoltage threshold level. For example, if N-channel transistors are used, a zero voltage or even slightly negative voltage on the gates of pull-up or pull-down devices may cause pull-up and/or pull-down N-channel devices to have a greater than allowable subthreshold leakage current. This problem is exacerbated by the very large pull-up and pull-down devices, which more readily draw current.
One attempt to address this leakage current problem was to couple an additional transistor to the pull-up transistor. The additional transistor had its source connected to the gate of the pull-up transistor, and the drain of the additional transistor was connected to the junction between the pull-up and pull-down transistors. The gate of the additional transistor was connected to ground. If the additional transistor was made with a lower threshold voltage than the pull-up transistor, it would turn on prior to the pull-up transistor. Therefore, if the DQ pad was driven to a negative voltage level, the additional transistor could prevent the pull-up transistor from turning on. However, the additional transistor does not sufficiently prevent leakage current from V.sub.cc through the pull-up transistor to the DQ pad.
Another problem arises when a DQ pad is driven to a negative voltage level. For example, if the negative voltage on the DQ pad is more negative than the substrate bias voltage, such that the difference between the voltages is equal to or in excess of a threshold voltage of the diode (barrier potential across a depletion layer of a diode), then the diode will conduct current. This leakage current is not desirable.
Therefore, it would be desirable to provide apparatus and methods for protecting against leakage current from V.sub.cc or ground to the DQ, or alternatively stated, across pull-up and/or pull-down devices. In particular, it would be desirable to provide apparatus and methods for reducing or preventing leakage current when the DQ pad is driven to a potential such that a leakage current exists.